Message text written by Anthony Rhodes
>I still do not understand the physical and operational differences of the
vacuum
port for advance and retard.
Do you know of a source that shows a vacuum vs throttle position graph. I
am
interested to see what the shape of the curve is and where the peak is....
I'd expect that the CD principle would cause the vacuum at the port to
have a plateau after a while as the throttle is opened (i.e. the piston,
not the
throttle position is regulating vacuum to the port.
Can you describe the graph in words?
<
I'm sure that there is a graphic representation of the vacuum to throttle
position relationship but I will have to look for it. As for the CD
principal: this is independent of this . The vacuum created by the venturi
is not enough to activate the module. The vacuum in the intake manifold is
the vacuum that operates these things. Throttle position is the imprtant
thing here as the vacuum is generated by the air passing the throttle
plate. As the throttle is rotated the bottom of the plate moves in toward
the engine and the top moves outward. A port on the bottom of the throttle
bore can be placed such that it is on the down stream side of the throttle
when the throttle is closed but as the bottom of the throttle plate moves
inward it moves past the port and now there is no vacuum at the port.
A port on the top of the bore cam be placed such that it is upstream of the
throttle when it is closed but as the top of the plate moves outward it
passes the port and suddenly vacuum is present at the port. This vacuum is
used to actuate the vacuum advance on some models and the EGR valve on
others.
Hope this helps
Dave
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